J/A+A/575/A121 Photodesorption of H2O, HDO and D2O ice (Arasa+, 2015)
Photodesorption of H2O, HDO, and D2O ice and its impact on fractionation.
Arasa C., Koning J., Kroes G.-Jan, Walsh C., van Dishoeck E.F.
<Astron. Astrophys. 575, A121 (2015)>
=2015A&A...575A.121A 2015A&A...575A.121A
ADC_Keywords: Atomic physics
Keywords: astrochemistry - molecular processes - ISM: molecules -
solid state: volatile
Abstract:
The HDO/H2O ratio measured in interstellar gas is often used to draw
conclusions on the formation and evolution of water in starforming
regions and, by comparison with cometary data, on the origin of water
on Earth. In cold cores and in the outer regions of protoplanetary
disks, an important source of gas-phase water comes from
photodesorption of water ice. This research note presents fitting
formulae for implementation in astrochemical models using previously
computed photodesorption electronicciencies for all water ice
isotopologues obtained with classical molecular dynamics simulations.
The results are used to investigate to what extent the gas-phase
HDO/H2O ratio reflects that present in the ice or whether
fractionation can occur during the photodesorption process.
Probabilities for the top four monolayers are presented for
photodesorption of X (X=H,D) atoms, OX radicals, and X2O and HDO
molecules following photodissociation of H2O, D2O, and HDO in
H2O amorphous ice at ice temperatures from 10-100K. Significant
isotope ects are found for all possible products: (1) H atom
photodesorption probabilities from H2O ice are larger than those for
D atom photodesorption from D2O ice by a factor of 1.1; the ratio of
H and D photodesorbed upon HDO photodissociation is a factor of 2.
This process will enrich the ice in deuterium atoms over time; (2) the
OD/OH photodesorption ratio upon D2O and H2O photodissociation is
on average a factor of 2, but the OD/OH photodesorption ratio upon HDO
photodissociation is almost constant at unity for all ice
temperatures; (3) D atoms are more ective in kicking out neighbouring
water molecules than H atoms. However, the ratio of the photodesorbed
HDO and H2O molecules is equal to the HDO/H2O ratio in the ice,
therefore, there is no isotope fractionation when HDO and H2O
photodesorb from the ice. Nevertheless, the enrichment of the ice in D
atoms due to photodesorption can over time lead to an enhanced
HDO/H2O ratio in the ice, and, when photodesorbed, also in the gas.
The extent to which the ortho/para ratio of H2O can be modified by
the photodesorption process is discussed briefly as well.
Description:
The data listed in this electronic table are the results of molecular
dynamics simulations of water ice photodissociation and subsequent
desorption mechanisms. Upon photoexcitation and dissociation of a
water molecule within an ice mantle frozen onto an interstellar or
circumstellar dust grain, there are several potential chemical
outcomes, the probabilities for which are dependent upon ice
temperature and ice monolayer. Here, we list the probabilities per
monolayer as a function of temperature for each outcome following the
dissociation of a H2O, HDO, or D2O molecule in water (H2O) ice.
DOH refers to the dissociation of HDO into D+OH and HOD to the
dissocation of HDO into H+OD; hence, the data contain probabilies for
4 dissociation events. These data have been compiled from the raw
simulation data which considers around 6000 trajectories or initial
conditions.
File Summary:
--------------------------------------------------------------------------------
FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
outcomes.dat 96 70 Probabilities as function of monolayer and ice
temperature for each possible outcome following
the dissociation of a H2O, HDO, or D2O
molecule in water (H2O) ice
table2.dat 35 100 Probabilities and associated errors as a function
of monolayer and ice temperature for H or D
atom desorption following the dissociation of a
H2O, HDO,or D2O molecule in water (H2O) ice
table4.dat 36 100 Probabilities and associated errors as a function
of monolayer and ice temperature for OH or OD
desorption following the dissociation of a H2O,
HDO,or D2O molecule in water (H2O) ice
table5.dat 52 100 Probabilities and associated errors as a function
of monolayer and ice temperature for XYO (X,Y=H,D)
desorption and H2O desorption via kick out
following the dissociation of a XYO molecule in
water (H2O) ice
--------------------------------------------------------------------------------
Byte-by-byte Description of file: outcomes.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 3 A3 --- Spec Species name (H2O, HOC, DOH or D2O)
6- 9 F4.1 K Tice [10/90] Ice temperature
12 I1 --- ML [1/6] Monolayer number
15- 24 E10.5 --- P1 [0/1] Probability for outcome 1 (1)
27- 36 E10.5 --- P2 [0/1] Probability for outcome 2 (1)
39- 48 E10.5 --- P3 [0/1] Probability for outcome 3 (1)
51- 60 E10.5 --- P4 [0/1] Probability for outcome 4 (1)
63- 72 E10.5 --- P5 [0/1] Probability for outcome 5 (1)
75- 84 E10.5 --- P6 [0/1] Probability for outcome 6 (1)
87- 96 E10.5 --- KO [0/1] Probability for kick out (1)
--------------------------------------------------------------------------------
Note (1): Definitions:
Outcome 1 = Xdes + OYtrap i.e. H/D desorbs and OH/OD is trapped
Outcome 2 = Xtrap + OYdes i.e. H/D is trapped and OH/OD desorbs
Outcome 3 = Xdes + OYdes i.e. both products desorb
Outcome 4 = XYOdes i.e. H/D and OH/OD recombine and desorb
(direct mechanism)
Outcome 5 = Xtrap + OYtrap i.e. both products trapped
Outcome 6 = XYOtrap i.e. H/D and OH/OD recombine and remain trapped
Kick out = H2Okick i.e. H/D kicks out a neighbouring H2O molecule
(indirect mechanism)
--------------------------------------------------------------------------------
Byte-by-byte Description of file: table2.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 4 F4.1 K Tice [10/90] Ice temperature
7- 9 A3 --- Sexc Photoexcited species (XYO where X,Y=H,D)
12 A1 --- Sdes Desorbed species (H or D)
15 A1 --- ML [1-4A] Monolayer number or average (G1)
18- 25 E8.3 --- Pdes [0/1] Probability for X atom desorption
following XYO dissociation
28- 35 E8.3 --- Edes [0.0028/0.0064] Associated error for
desorption probability
--------------------------------------------------------------------------------
Byte-by-byte Description of file: table4.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 4 F4.1 K Tice [10/90] Ice temperature
7- 9 A3 --- Sexc Photoexcited species (XYO where X,Y=H,D)
12- 13 A2 --- Sdes Desorbed species (OH or OD)
16 A1 --- ML [1-4A] Monolayer number or average (G1)
19- 26 E8.3 --- Pdes [0/1] Probability for OX desorption following
XYO dissociation
29- 36 E8.3 --- Edes [0/0.0033] Associated error for desorption
probability
--------------------------------------------------------------------------------
Byte-by-byte Description of file: table5.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 4 F4.1 K Tice [10/90] Ice temperature
7- 9 A3 --- Sexc Photoexcited species (XYO where X,Y=H,D)
12 A1 --- ML [1-4A] Monolayer number or average (G1)
15- 22 E8.3 --- Pdes [0/1] Probability for XYO desorption following
XYO dissociation
25- 32 E8.3 --- Edes [0/0.0013] Associated error for desorption
probability
35- 42 E8.3 --- Pkick [0.0002/0.0235] Probability for H2O
desorption via the kick out mechanism
following XYO dissociation
45- 52 E8.3 --- Ekick [0.0002/0.0019] Associated error for kickout
probability
--------------------------------------------------------------------------------
Global notes:
Note (G1): The average values over the top four monolayers for each ice
temperature and for each probability are also listed and indicated with
an "A".
--------------------------------------------------------------------------------
Acknowledgements:
Catherine Walsh, cwalsh(at)strw.leidenuniv.nl
(End) C. Walsh [Leiden Obs., The Netherlands], P. Vannier [CDS] 25-Mar-2015